Electrical switching system



Jan. 19, 1954 T. H. FLOWERS 2,666,809

ELECTRICAL SWITCHING SYSTEM Filed Oct. 26. 1948 6 Sheets-Sheet l BY m w 7 ATTORNEYSi Jan., 19, 1954 T. H. FLOWERS 2,666,809

ELECTRICAL SWTCHING SYSTEM Filed Oct. 26. 1948 6 Sheets-Sheet 2 DELAY .l/VE

ATTORNEYS` J. i9, 1954 T. H. FLOWERS 2,666,809

ELECTRICAL SWITCHING SYSTEM Filed Oct. 26. 1948 6 SheetS-Sheel 5 Jan. 19, 1954 T. H. FLOWERS ELECTRICAL SWITCHING SYSTEM 6 Sheets-Sheet 4 Filed OCT.. 26. 1948 Jan. 19, 1954 T. H. FLowl-:Rs

ELECTRICAL swITcHING SYSTEM 6 Sheets-Sheet 5 Filed Oct. 26. 1.948

6, y mi n, ,/M. W u w .m/ n@ Jan. 19, 1954 T. H. FLOWERS 2,666,809

ELECTRICAL swITcHING SYSTEM Patented Jan. 19, 1954 Uiiii'iED STATES PATENT `OFFICE ELECTRICAL SWITCHING SYSTEM Thomas Harold Flowers, London, England I Application October 26, 1948, Serial No. 56,619

Claims priority, application 'Great Britain `October 27, 1947 This inveutieii relates to electrical communication systems, that is to electrical systems for conveying speech frequency or other signals.

An object ci the invention is to provide an` 1mproved communication system in which t1mespaced signals are employed to convey information.

Reference will Toe made both to impulses and pulses by impulses is meant Aalcrief change of current in a 'circuit while pulses will Vloe used to d fine a rhythmic train of impulses.

The invention is especially adapted to automatic telephone exchange systems. In known such systems electricallyy controlled mechanical devices are employed to connect any one 'of a plurality of Z-Wire or 4-wire speech circuits With any one ci a plurality of 'other circuits or trunks and such connections may r`be made simultaneousl'y.

The present invention is concerned with making conn uions between one or vmore trunks and a common signal circuit adapted to transmit information to the vtrunks and has 'for an object to provide improved means for connecting the common signal circuit with Aa trunk.

i further object of the invention is to provide impur-JerA ymeans for connecting any 'one of a plura f of circuits with any one of a plurality 3f tiLli Other objects and features of the invention will he Amade clear from the following description `but in particular the invention Ycomprises in combination a switching usystem, time division multiplex apparatus providing time spaced signals, a common signal circuit, means for `applying said time spaced signals to said common signal circuit, signal suppression 'gate circuit means connected -to Said common signal circuit, fa plurality Vof timespaced signalf'sele'cting means,

"rst and second circuits vfor feeding "said time spaced signals to said selecting means, means operating in response `to 'signals in said circuit to produce a pulse in a lp'rec'leterrnined one of said se ccting means, rr'ieans 4for 'connecting said first circuit to said suppression gate circuit means, a `thi-rd circuit lfor yconnecting together said selecting means and said signal suppression te circuit "and-means 4for applying said p to said third circuit in such 'a sense that said pulse suppresses transmission of Isaid `time spaced signals through said suppifesion gate cirt to l'said first circuit. "Communication e cir-'uit 'means are included in said second circuit and means are provided for applying said pulse to "said 'communication gate -circuit means.

A switching system according to the invention also comprises uinjection gate circuit means con-- nected in said rst `circuit and a sourceof vmarking signals connected to said 'injection gate 'circuit means which operate to apply time spaced signals to rsaid selecting means only on coincidence at 'said injection gate means of time `spaced signals and said marking signals. A master source of pulses is provided the impulses vof which are synchronous With sai'd'time spacedsinals and synchronising gate vcircuit means operate to place said Selecting means in communication with said master source of pulses when 4an impulse is present in said third circuit.

The invention further consists in Y'a switching system which comprises means `for producing timespaced `electrical impulses, 'a plurality of calling circuits arranged to transmit auditif-fre# quency signals, means for modulating lsaid iinpulses by said audio-'frequency Asignals so that the signals from any 'one of said calling rcircuits modulate impulses separated one from the other 'by predetermined fixed time-intervals, 'a common signal circuit l arranged to 'transmit the modulated impulses from all said calling circuits, -a plurality of trunks, a plurality of communication gate circuit means each connected with 'one of said trunks and'a'rran'ged to control the Aconnection of a trunk with said common signal circuit, means for intermittently operating each of said vcommunication gate circuit means so that the trunk connected to an operated ygate circuit means is operatively connected with said common signal circuit for transmission of signals on said common signal circuit to said trunk at such times as impulses lcorresponding to signals on one of said ncalling circuits appear on said common signal circuit and means for demodulating the impulses received by a trunk whereby audiofreouen'cy signals are transmitted along the trunk.

The invention also provides a switching system wherein audio-frequency signals appearing on any one of a plurality of speech circuits all `adapted to be connected simultaneously with a Common signal circuit are caused to appear on the common signal circuit as time-spaced impulses and are transmitted over any selected one `0f a Blurality-of trunks which is connected with the common signal circuit -by-.gate circuit means operated to connect the `common signal circuit'opveratively with said selected trunk 'at such times as impulses correspc'nding to signals von one speech circuit appear `on the said common signal circuit.

The invention is illustrated by way of example in the accompanying drawings in which:

Fig. 1 is a schematic diagram illustrating an arrangement which embodies pulse selector means according to the invention for connecting a plurality of circuits via a common signal circuit with a plurality of trunks by the method known as line nding e. g. the selection of a calling circuit from among a plurality of circuits. This arrangement will be called a line nder.

Fig. 2 is a schematic diagram illustrating a further arrangement for connecting a plurality of circuits via a common signal circuit with a plurality of trunks by the method known as selecting e. g. the selection of a non-calling or quiescent circuit from among a plurality of circuits. This arrangement will be called a selector.

Figs. 3a., 3b, 3c and 3d together show an arrangement of an automatic telephone exchange embodying the invention and Fig. 4 is a timing diagram which illustrates the relation of pulses produced by the arrangement of Figs. 3a to 3d.

Referring to the accompanying drawings TI I and TI I represent respectively the transmit and receive 2wire channels of a 4-wire circuit which is one of a plurality of l-wire circuits which may be connected for example to subscribers lines or to trunks connected by the circuits to trunks of a different switch. TI2 and TI2 represent the transmit and receive 2wire channels of a second 4-Wire circuit in the plurality of 4wire circuits, the arrows indicating the direction of transmission in the case of each channel. When any one of the plurality of 4-wire crcuits passes from the free to the engaged condition, that is, it calls, the calling circuit is connected, as will be described, to a free 4-wire trunk in a plurality of l-wire trunks of which one trunk having transmit and receive 2wire channels TZI and TZI respectively is shown. The 2wire transmit channels TI I, TI2 are each connected to a time-division multiplex transmission system which produces on a common signal circuit II time-spaced signal impulses corresponding in time order t0 the channels 'II I, TI2 and so on. If it be assumed that there are one hundred channels such as TI I, that the time-division multiplex system H20 provides one hundred channels which are allocated one for each of the 2wire transmit channels TI I, TI2 that the pulse repetition frequency on each multiplex channel is 10,000 per second, then the impulses appearing on the common signal circuit IOI will be spaced at l microsecond intervals. In the example now being described it is arranged that when a 2wire transmit channel is engaged, the corresponding impulses emitted from the multiplex system |00 will have a nite mean amplitude, speech currents being transmitted by modulation of the impulses but when the transmit circuit is idle the impulses will fall to Zero or substantially zero amplitude. The 2wire receive channels are each connected to a one hundred channel time-division multiplex transmission system 200 which distributes time spaced signals 0n a common signal circuit 20| in time order to the channels TI I', TI2', via low pass filters II2", |22 which demodulate the impulses to audio-frequency outputs which are amplified vbythe ampliers IIS', |23', before being -passed over the channels TI I', TI2

The time spacing control means of the multiplex systems |00 and 200 are locked together such that the time element of system |00 corresponding to the 2wire transmit channel of a l-wire circuit is coincident with the time element of the system 200 corresponding to the 2wire receive channel of the same 4-Wire circuit. The common signal circuit IOI is commoned to a plurality of communication gate circuits of which one I02 is shown and of which there is provided one for each of the 2wire transmit channels of lg-wire trunks which form a plurality of 4-wire trunks. Only one of these 2wire transmit channels is shown and is designated T2I. The corresponding 2wire receive channel is shown as TZI and is connected via a communication gate circuit 202 to the common signal circuit 20|. The communication gate circuits of other 2wire receive channels of the ds-wire trunks are similarly connected to circuit 20I. The communication gate circuit |02 is opened by and for the duration of an impulse over lead I03 and communication gate circuit 202 is opened by and for the duration of an impulse over lead 203. Leads |03 and 203 are both connected to lead 30| over which, as will be described, are passed impulses coincident with the channel impulses of any one of the circuits of the plurality of 4-wire circuits to which connection is required.

The input to a 2wire channel e. g. 'I'II in the transmit direction and to 'I2I in the receive direction comprises a D. C. signal which is amplitude modulated by A. C. speech and other signais. In the absence of an input the corresponding channel impulses in the multiplex are of substantially zero amplitude. The D. C. component of an input causes the channel impulses to assume a standard amplitude said channel impulses being modulated by the A. C. component of the input. Demodulation of the channel impulses in the transmit direction by the low-pass lter 400 produces a modulated D. C. signal which is a replica of the input except that it is greatly attenuated. The amplifier 500 could amplify both the D. C. and the A. C. components but in practice this is undesirable. The amplifier 500 therefore amplies the A. C. component which it delivers to the transmit channel T2 I. The apparatus marked 802 demodulates the channel im pulses and amplifles the D. C. component, the D. C. and A. C. outputs being recombined at the transmit channel TZI. In the reverse direction of transmission the same arrangement is used to demodulate and amplify the channel impulses from the multiplex 200, the low-pass illters I "l' 2.12' and ampliers II3, |23 handling the A. C. components and the apparatus III,

I2I' the D. C. components.

A line finder one of 'which is provided for each 4-wire trunk in the plurality of 4wire trunks comprises a trigger circuit 300 which is, per se, of the well-known type which has two positions of electrical stability. This trigger circuit is set to one or other of its two positions of stability by impulses. The circuit 300 is set from its rst to its second position by an impulse over the leads from which the rimpulse will emerge, 'in' fthe Sexample now described, after a time-delay-'of 29936 microseconds, plus or minus la tolerance of, for example, one-fifth of `a microsecond. The impulse from the delay line is passed to a synchronising gate circuit 3Q? over lead 386. fConne'cted to the gate circuitifi overleadt is a'source of impulses K! 'each of duration, ior example, `onelifth microsecond, spaced one microsecondapait and substantially coincident with the channel impulses.

An impulse KI will occur approximately at the time centre of an impulse received over lead Sd and, during the Kl impulse an impulse Tis emitted over lead 353 to set the 'trigger circuit 35d from its /rst to its second position fand thus to initiate a 'further impulse from said trigger circuit over lead 39s as already described. The process is thus repeated by which the trigger'cir-A cuit 313i? is Vchanged from fone of .its positions to the other. The trigger Vcircuit 359, 'delay line' 305 and gate circuit Sti together with'their associated circuits are thus a circulating system which when once started produces an impulse 'over lead Sill every one hundred microseconds to operate on the communication gate `circuits I' and 2:02. The K! impuls-es over lead '383 are synchronised with the multiplex system l il@ and 'Zilli so that the gate circuits H32 and 21H2 are'opened as described at times coinciding with the appearance of the channel impulses of one v2l-ivire circuit in each of the multiplex systems. The impulses passed through gate circuit IGZ are Ipassed through the low-pass lter 480 for demcdulation to 'audio frequency which is amplified by amplier aus and passed to transmit channel T21 as valready described. Audio frequency vcurrents incoming on receive channel TSI' are modulated to channel impulses by the communication gate circuit 202 which is opened by the impulses appearing over lead gt3 and the modulated channel impulses are commoned with the modulated 1channel impulses from other engaged trunks to the common sig'- nal circuit 2li l. It will th-us be apparent that any ll-wire circuit, for example T l, TH in the plurality of Ll-wire circuits, can be electrically connected to any one of the Zl-wire trunks, 'for example T2l, T2 l in the plurality of 4-'wire trunks provided that the circulating system associated with said any one ll--wire trunk as described is arranged to emit over lead 3&1 'impulses corresponding in time With the channel impulses `of any selected one of the fl-Wire circuits connected to the multiplex systems l-S and 28E.

When no circuit in the plurality of circuits is Y engaged, all impulses over lead it?! will vthen have zero amplitude. If, now, one of the circuits for example Ti l, Tl l becomes engaged that is, it calls, impulses will appear over the 'common signal circuit l Si in time position relation which corresponds to the calling circuit. It has already been stated that the duration vand spacing of the impulses have prescribed dimensions and it will be appreciated that the time Ybetween the channel impulses which correspond to one circuit is occupied by channel impulses which correspond to the various other circuits so that, if more than one circuit is engaged at a time, a number of channel pulses will appear simultaneously on the common signal circuit lill, these pulses being formed by the impulses which have 'been described and which appear one after the `other on the common signal circuit. For the purpose of this explanation only one pulse is being conoiie'f the lcircuits for example I'Iilriq. 'T1 I" 1 calls. The common signal circuit Iul iis connected over lead l-I to ya gate circuit 100 which is normally open to allow impulses over le'a'd l1-i) to pass L"over lead F101! and thence via leads Go2 to pulse injection gate circuits tot or which there is one to ei/'ery such as T21, T21". Gate circuit 1mais closed by impulses which are :recei'v'ed over lead 102, :these impulses being 'sup'- plied to -the lead 11102 over the delay lines 365 which `are individually-associated with the trunks suchas T221., T21' l. n Gate circuit 609 allows impulses received overthecommon lead 'l0-'l and lead 632 to pass to 'the trigger circuit '31m over lead only 'when a-markingisig'n'al from va trunk marker is applied 'to the gatecir'cuit- 600 'over lead Sel. The 'trunk inarker is adapted, by means not shown, to Vmark only one gate circuit stl) at a time and that only 'a Vgate circuit which 'associated with Ja free trunk iin the plurality of 4- wire trunks. impulse `of the pulse which lappears over the common Isig-nal circuit il when one `of the circuits lfor example Tl I, TH" calls passes rover lead 12H1, through `gate -circuit 'me which is open at this time, through ya gatech'- cuit Een marked by the trunk marker, over llead 362 and thuis s'ets 'trigger Circuit 'S'li'from its first to its second position, thus starting the circulatng system 'as described above. The next and subsequent impulses Ito appear over common signal circuit `I lli fromthe-'callin'g' circuit coincide with the impulses V'emitted over lead 301 and thus for the duration *of tl-'reinpulse's the engaged circuit iselectrically connected to the marked trunk.

I The lsaid next 'and subsequent impulses will be prevented 'from 'pass-ing `Siate which is thus impulses which appear coincidentally with'them over lead W2 from le'ad 33'09 and thus cannot cause another circulating system to start gen eratmg impulses. Whenlthe connection has been established as described 'the C. component o deinodulation obtained rfrom the demodulatiin'g apparatus S02 'is passed over lead'lS to the trunk marker to indicate that the marked trunk `and its line runder .has been taken into service. rthe trunk 'marker then lm'arks another fr'e'e trunk which will connectitsellf as just described to the next circuit lo'f fthe plurality Oicircuits `to call. As the removal of 'the :marking over lead ein is lrelatively slow, shouifd two circuits cali 'and start their Ychanmel pulses wit-hin Aair/ery short .interval `intime there would be .a danger of the two calling circuits becoming connected to the same trunk. For this reason the impulses emitted 'from 'the trigger vcircuit 30a .are .passed over lead etc to au integrating circuit 96! Vwhich converts the impulses into a continuous signal which is applied over Alea-d 892 to the gate Suu so as to close this gate as long as the signal persists and this continuous signal .is applied in advance of the remetal or the marking or the trunk marker.

'When a call-ing circuit iis released, the `corre1 sponding pulse on the common signal circuit l il! becomes Zero thereforev the output impulses from gate m2 and the output from the apparatus 392 also becomeszero. .A lead .from the demcdulating armaratus `23112 is connected to an impulse generator tite which is adapted to produce a long impulse over lead 813i when the impulses .rrom

v:gate lll-2 vand Vthe output y:trcmn apparatus i392 cease. The .impulse over leadtl holds the 'trie k:ger circuit .acuin'its urst position despite an im* pulseiwhich `appears vover .lead 393 and which, in

sidered that is, the pulse which appears when 75 the Aabsence ci thempulse over lead 801, Would set the trigger circuit 300 in itssecondfposition. The circulating system is thusginterrupted and the connection between circuit and trunk is broken.

The arrangement shown in Fig. 2 differs from that shown in Fig. 1 in that the lead |I0 is not connected to lead IOI but to a line marker, and the device 800 is not connected to the apparatus 802 but to a controlling lead |000 to which lead I is also connected instead of to a trunk marker. The arrangement of Fig. 2 functions `to connect a 4-wire trunk T2 I, T2I' vwhich has been marked by a signal over lead |000 `from a previous switching stage to the selected circuit such as, for example, the circuit Tl I, TI I. A line marker, not shown, is caused to emit a pulse over lead ||0 coincident with the channel pulse corresponding to the circuit TI I, TI I and-thus to start the circulating system associated with trunk T2 I, T2I at the appropriate time. The desired connection is thus established and is broken down when the signal on lead |000 is removed.

Referring to Figs. 3a, 3b, 3c, 3d, and 4, Figs. 3a to 3d show the circuit arrangement of a simple ten-line automatic telephone exchange which would be suitable for a private exchange serving subscribers located close together for example in the same building.

A subscribers exchange equipment is shown as consisting of the lines T and L together with the apparatus shown between these lines and t'ne valves I and II on Fig. 3a. A second subscribers line is similarly arranged and connected to the valves 2 and I2 but has been omitted from the drawing. The valves 3 and I3,.4 and I4 I0 and 20 are also connected to subscribers through similar' apparatus. At each subscribers station is a conventional handset consisting of a carbon granule type transmitter and electromagnetic receiver, a switch hook to give the D. C. calling and clearing conditions and an impulsing dial, all connected to a pair or circuit T.A In addition each subscriber is provided with a signalling device, not shown, for example a lamp and battery connected to the pair L.

The valves I to I0 are arranged to form pulse modulators and serve, with the associated apparatus to be described as a multiplexing device to convert, in a time division multiplex process, the transmitted D. C. and audio frequency signais which appear on the pairs T into amplitude modulated time-spaced signals on a common signal circuit, i. e. the common anode circuit of the valves I to I0. After polarity inversion by a valve 3I and power amplification by a cathode follower valve 33 the time-spaced signals are transmitted over the lead G to a plurality of first selectors or, as they will be termed henceforth herein, line iinders, the circuit of one of these line finders being shown in Fig. 3c.

Each line finder as shown in Fig. 3c, is trunked direct to a selector, there thus being a number of selectors and the circuit diagram of such a selector is shown in Fig. 3d.

Valves II to 20, shown in Fig. 3a, provide the means whereby the received speech or signals which appear on the common signal circuit R as amplitude modulated time-spaced signals, are distributed to the exchange circuits of the subscribers. y.

Each line finder is associated with in addition to means enabling it to connect to a calling subscriber by line finder action, apparatus arranged to respond to the dialled number of the wanted subscriber and, also to complete the connection.

-The marking of a disengaged line finder, and thus the marking of a free trunk, is performed by a trunk marker LFA, the circuit of which is shown in Fig. 3c to the right of the broken vertical line.

Both the line nders and the selectors are connected to the common signal circuits G and R via gate valves such as valves 4I and l2 shown in Fig. 3c.

As has already been described information from a calling circuit, the subscribers circuit in the arrangement now being described, is converted into a pulse which consists of regular timespaced impulses. The interval between the time centres of adjacent impulses of one of these pulses may be called the multiplex period the reciprocal of the period being the pulse repetition frequency and the available time during which an impulse may persist is called the channel spacing, the term channel referring to a path through a multiplexing device. The pulse repetition frequency must exceed twice the highest audio frequency used to modulate the impulses in order to obtain satisfactory transmission.

In the accompanying drawings the screen grids of pentode valves are shown disconnected and it is to be understood that the leads to these grids have been omitted to avoid confusion and that they are to be connected in the normal manner to a source of potential which is suitable for the operating conditions and characteristics of the valves.

As stated above ten circuits are provided in the exchange shown in the drawings. The multiplex lpulse repetition frequency employed is 10,000 cycles per second so that each circuit may theoretically be connected to the common signal circuits for a channel period of ten microseconds during each multiplex period of microseconds. In practice, however, due to the necessity for ensuring that each circuit is disconnected before the next is connected to the common signal circuit, and due to the limiting speeds at which the switching operations can be carried out, it is advantageous to arrange that each circuit is connected to the common signal circuits for only a fraction of the channel spacing. A suitable fraction is one fth and in the following description it is assumed that the duration of a channel impulse corresponding to one circuit is 2 microseconds.

The valves I and II, 2 and I2, etc. are switched oi, i. e. made non-conducting over their anodes, by a bias voltage V5 which is applied via resistors such as R6 to the suppressor grids of the valves. The voltage V5 is sufficiently negative with respect to the cathode supply voltage V3 to cut off anode current. The valves may be switched on by, and for the duration of. positive-going impulses of a magnitude substantially equal to the difference between the voltages V3 and V5 applied to the suppressor grids via condensers such as C6.

The positive-going impulses applied to the condensers such as CG are obtained from a pulse generator PG. This generator may be designed in accordance with well-known practice and any suitable arrangement may be employed. The generator is required to generate pulses on twelve output-leads marked KI, K2, PI, P2 PIO. The-pulses generated are illustrated by the tim- Ying diagram of Fig. 4. The lead KI carries a continuous pulse which will be called the KI pulse. which consists of positive-going impulses ege asseof substantially rectangularf-fo`rmeafch endure' ing for two microseconds and occurring with a repetition frequency ofv 1 00g000 cycles per second. riShis Ki pulse is the primary timing" means of the system and may be obtained bythe well' known method ofsquaring the sinusoidal output of a 100,000L cyclesper' o nd oscillator and then" diferentiatin'g andre quarilig toobtair thedesired asymmetrical waveform. The head' K2 is supplied with thev K pulse whichl consists oi negativeegoing impulses ofthe same forni, dura;-v tion and repetition frequency asY the Klpulse but arranged to occur approximately'eightmicro: seconds later.V Itlnay' be'obtained by using the saine source of sinusoidal waves asf isfllsedl for the Kl pulse, said waves being p a`s sed` through aA suitable phase shifting network b'e'foieheing subjected to the proce's'sfof squaring, differentiatv ing, re-squaring andpolaritylinversion necessary to produce the requiredwaveform. The"v lead Pi is supplied with the: PI4 pulse wlflich` may be obtained by gating every tent-h impulse' te the Pi lead. Thisirnaybedone by using asource oi sine waves of frequency 10,000 cyclesjperse ond synchronised with the 100,000 cycles Vper sec-v ond oscillator and suitably phased, squared, differentiated, and resquared toenable it to operate on a gate-ampliiier situated loetvlreen'V the Ki lead and the Plv lead; Suchy a gate-ampliiier may bel a pentode valve arrangedin known manner to conduct onlyat the desired" times. Another method of producing the Pl pulse may consist in employing ai conventional electronic iai-quinary counter served at the input with the Ki pulse and emittingV at its' output a square wave which after dieren'tiation may be employed to operate on a gate-amplifier situated between the EZ! lead and tliePlV lead. The individual impulses on the PI lead are thus synchronous with, and are of similar polarity, duration and form as one and every tenth Ki impulse. The lead P2 is supplied with the P2l pulse which is similar in all respects tothe PIi pulse except that the P2 impulses are delayed ten microseconds with respect tothe Pi impulses. The P2 pulse may be generated as in the lirst method suggested for thePi pulse but with a suitable phase shift circuit inserted in the 10,000 cycles per second drive circuit. Similarly the P3-Pl0 pulses are supplied' on' the appropriate leads, each being derived intime relation to the lower numbered pulse as' suggested aboveforderiving the P2 pulse in relation to the PI pulse.

As shown in Fig.' 3o, the ahodes of Valves l to l@ are oonnectedito a commonN anode resistor Resistor R4" and condenser C4 are provided for de-coupling the' supply potential VI which is suitably positive with respect to V3.r

When subscribers are disch-'raged their circuits 'i' are open circuited' and under these condi` tions the control gridsl'of valves! to it are' at earth potential. Theca'thodes'of these valvesI are connected Via resistors such'. as R5 to a( supply potential V3, positive'with respect to earth such that all the valves arecut ofi Vat theircontrol grids. Thus, in` spite of'the impil lses which'are applied from generatorfPGto' thesuppres'so'r" grids of the Valves, no impulses'are emitted into the common anodecilfc'uit 'whnthe'channels are disengaged. Should afsub'scriber Anow engage his line, direct currentiiowsffrom the positive ter`v minal of the exchan'gebattery B \via"one half of the primary win'ding'Uof the hybrid trans former TI, oneleg' of tliepair T thfrough the' subscribers transrriitter," dialL and switch-hook contacts, back along the other leg" of the pair '1 and the other half of the primary winding of transformer TI, through the resistor R to the earthedterminal of the exchange battery. An eiect of the feeding current thus drawn is to raise the potential of the junction of Ri and the primary winding of transformer 'li' from that of earth to the value at' which the rectifier element Wi conducts andso arrests the potential rise aft a value very slightly positive with respect' to' t'liefD'. C; supply voltagev V31 This rise in potential isi communicated via the resistor R2 to the control grid ofthevalve l, and' such is the value' chosenv for thecathode resistor R5 that inthis conditionthe valve i operates satisfactorily as" an amplifier: Similarly are the valves 2 to I 0' controlled' by tlie'act of engaging the lines' asso' c i'ated with them., ,When therefore', any su -L' scriber engages his line; a' negative-going channel pulse is developed across the c`oilo'n'.'1 c `ln'.anodeV resistor R3. This channel pulse isfdevelopdby. and is therefore synchronousl with th'efPi Yt P 'f' pulse applied to thesu'ppressor grid of the p afr ticular valve l' to l0' associated with that subi scrber. This feature of the iiiveritioi1'- ltis't'i-fv tutes the calling and holding signal from a call'- ing subscriber and, as Willbe seen, the arsi'lver` ingv signal from a called subscribe`i". 4 in likemanne'r when' other subscribers engage" their lines they too calise channel pulses to'be" developed, across resistor R3, which, apart troni polarity and amplitude are replicasv f the PT to' Pi' pulses which haveV been allocated totlieirl.'v

when a subseiflbeiaisenefages his line', the' po; tential at the control grid of the assbelated` valve i to ic' reduces to efarthpotential.' thus cutting eff' the valve and removing' trie associated chan"-V nel pulse from among those; if any, appearing across trie resister Rs. This feature' constitutes' the clearingfsig'na'l.

The' amplitude of any particular channel puisef is dependent on the po'te'lltial oi the control grid of the asseeiatedvalve l te le; r'tlie' ab's'ef'i o f any potential across the 'resistor R2, the chanf nel pulse 'will have a steady, amplitude v \`1`l1'i ':h 'willl be referred to as the standard arnllitude.A SpeechV orsisnalsincoming ori theipair T and develo ing alternating currents' iii1 the primary wird' ings' or transfo'rmerTl, will produc'egan alter-fV mating potential in the see'eiiaarywiiidiags er Ti; A part of this potential is,` via condenser 'C2-, de# Velop'ed across the resistor R2 and so will am? plituae modulate the channel impulses" ei'i'i'itted from' the anode 'of trie aes'ejciated valve'y l` te" l'oj. This is the means byywhien speech or' A. C. sig; nais are transmitteaitlir'cuin the multiplex;

Dallihg impulses are transmitted ih the saine: way as the calling and clearing signals, namely each time a suhscribers li'neis operi cifrcuited the impulses of the associated channel pulse duces to zero amplitude' and v vl'le'rfrthe liiif"i s re' closed the' impulses' reemiiien'edj As will be" seen, timing circuit means are provided on tire` selector -for differentiating between" dialling v and clearing'signals. channel impulses appeariiig acides' las aie polarity'-ilverted'- yby the ampliiier 3|' and' their communicated Viajconfdensers CT and Cil'to tl'itfA control' grids of the cath'dde followers 32` and 33T Valves 32 and :B3 have 'their ahodes conrleljted to' the supply potential Vl and their catlicdcs con nec'tei' through fcathodeleresistors to" earth.v The central grigi of valve 32 j is biasesV vialiesiisjtwf Bland Ri; if).,iheDtfQtsupplyp iefiiialzvi the control grid of valve `33 biass'ed'via R'S'to'v theYV l1 same voltage V4, which is made negative with respect to earth by an amount sufcient to hold valves 32 and 33 close to the cut-off point thereby enabling them to handle satisfactorily the positive-going channel impulses. The cathode follower 32 applies the impulses to the lead C, and the cathode follower similarly serves the lead G.

So far only the transmission of speech and signals incoming from the subscriber has been considered, and it has been explained that such speech and signals appear on the common signal circuit G in the form of amplitude modulations superimposed on the standard amplitude of the channel impulses. Speech or signals transmitted to the subscriber appear on the common signal circuit R as amplitude modulations superimposed on channel impulses of standard amplitude. As will be seen, these impulses are formed and modulated in the line nder or selector to which the subscriber is connected and are synchronous with the channel impulses on the common signal circuit G. Appearing on the common signal circuit R as negative-going impulses, these impulses are polarity inverted by the amplifier 34 and communicated to the common control grids of the valves I I to 20. These valves have their cathodes connected, each through a cathode resistor, to the supply voltage V3, and the control grids through a common grid leak to earth, the cathode resistor being so chosen with regard to the valve characteristics, the voltage V3 and the standard amplitude of the positive-going channel impulses that each channel impulse raises the potential of the control grids into the operating range of potential within which the valves amplify satisfactorily. Valves II to 20 have the suppressor grids connected to those of valves I to I respectively and are therefore subject to the same operation of being cyclically switched on for the duration of the impulses supplied over leads PI to PIU Since, as has been said, the appearances on the common signal circuits G and R of any particular channel impulse are synchronous it follows that the channel impulse on the common signal circuit R will be gated through the appropriate valve II to and not through any other. As will be described later the channel impulse on th common signal circuit R does not appear until the called subscriber answers. When this occurs, negative-going channel impulses appear at the anode of the relevant valve of valves I I to 20 and the condenser C9 and control grid of the valve 35 rapidly becomes charged negatively via the diode DI sufficiently to cut off valve 35 and release the normally operated relay S, which via contacts SI, completes the circuit between the legs of pair L. This is the answering signal already mentioned.

The anodes of valves I I to 20 are also each connected to a low-pass filter II2' having a cut-off frequency slightly less than 5,000 C. P. S. Speech or voice-frequency signals existing as a modulation on the channel pulses pass the filter and are then amplified by valve I I3 and transmitted via transformers T2 and TI to the subscribers line. The purpose of the amplifier I I3 is to make good the losses due, amongst other things, to multiplexing. The loss due to multiplexing a ten channel system, using rectangular pulses of only one fifth of the channel period is 2499/2500 of the input power.

The arrangement of the windings of transformer TI will be clearly understood by those accustomed to telephone practice. Condenser CI is made large enough to provide negligible impedance to voice frequencies compared with that offered by the primary windings of TI, and ZB is a balancing network. It should be understood that the system has obvious inherent possibilities of amplification which, although requiring an accurate balance impedance, ZB, open up possibilities of longer, or cheaper, local lines.

A number of line-nders, equal to the number of simultaneous conversations required and arranged as shown in Fig. 3c is provided. The common leads to which all line finders are directly joined are as shown on the left-hand side of the diagram. On the top right hand side of the broken line of Fig. 3c is shown the circuit of the trunk marker LFA, this comprising a self-drive uniselector SA of well known type. The uniselector SA possesses two wipers each working over a set of contacts. One wiper is arranged to be connected to earth and the other via the driving magnet, SA, and associated mechanically operated interrupter contacts, SA dm., to the exchange battery. The trunk marker LFA is joined to each line-finder by two leads, such as W and X, which are wired to corresponding contacts in the two banks. The trunk marker is so adjusted that if the W lead to the bank contact on which the uniselector SA is standing becomes earthed the wipers will automatically step on until such a contact is found free from earth.

Lead W is connected to make contact EI of the relay E, the other side of the contact being earthed. As will be seen, while a selector is engaged relay E is operated thus preventing the uniselector SA from remaining on the relevant W contact. In consequence the trunk marker wipers will always be found standing on the X and W contacts of a disengaged line-finder, assuming there to be such a line finder. In this condition the trunk marker is said to mark that iine finder to take the next call, a function performed by extending earth potential over lead X to the resistors R43 and R44 in the line finder. When this line finder later becomes engaged, the wipers step on to mark another disengaged line finder.

In Fig. 3c valves 4I and 43, and 42 and 44 are the communication gates by means of which line finders are connected to the common signal circuit G and R respectively i. e. are the gates |02 and 202 of Fig. l. When a line finder is disengaged these gates remain closed, that is to say, the valves 4I and 42 are rendered inoperative. When a line nder becomes engaged these valves are cyclically rendered operative coincidently with the appearance on the common signal circuit G of the engaging channel pulse. The timing device preferred for cyclically operating the communication gates comprises a circulation system into which a single impulse, of the channel pulse it is desired to select, may be injected and which contains a delay device having a delay approximately equal to the multiplex period. 'I'he circulation system is so arranged that once started, impulses will circulate until deliberately stopped, and at each occasion of passing the injection point in the system will effect the required operation of the communication gates.

The delay device employed in the circuit of Fig. 3c is a mercury lled, supersonic delay line DL of the type now familiar to workers in many fields and particularly to those engaged on computer systems. Some devices are described in an article entitled An Ultrasonic Memory Unit for the E. D. S. A. C. by M. V. Wilkes and W. Renwick which appeared in the periodical Elecd.. A .s I

13 tronic Engineering vol. 20, No. "245, July 1948, and in an article entitled Mercury Delay Lines by T. K. Sharpless which appeared in the periodical Electronics dated November 1947, at page 134.

It will be appreciated by those skilled in the art that practical difiiculties arise when at tempts are made to construct a multiplicity of timing units, such as the mercury lled delay line, each so stable with respect to the master timing device, namely the KI pulse, that when. started said units will remain accurately synchronous with the KI pulse .for a long period. lt is possible, however, and means will be described to overcome these difficulties by employing a source of synchronising pulses andi using these to re-pl1ase the circulating pulse each time it reaches the injection point and so by preventing the cumulative effect of the ldrift in the delay line, limit the error to that of a single transmission through the line. ln this condition, the delay stability of the mercury delay line is adequate for the purpose required.

Valves 4t and 45 of Fig. 3c are the impulse injection and synchronising gates respectively. A negative-going impulse emitted from the comm mon anode circuit of these valves is arranged to be communicated to the suppressor grid of the normally conducting valve 46 thus changing over the trigger valves 46 and 41 from the rst position of stability to the second. The anode circuits of the trigger valves 46 and 4'! are connected to a balanced static relay comprising the rectifying elements W4, W5, W and Wl and associated transformers and resistors in such a manner that when the trigger is in thefirst position of stability the attenuation between a carrier frequency oscillator CG and an amplifier valve 43 is very great and when the trigger is in the second position of stability said attenuation is rendered small. In consequence, therefore, of changing over the trigger, carrier frequency is admitted to the amplier 48 until the trigger is changed back. This occurs 8 micro-seconds later by virtue of the incidence at the suppressor grid oi valve 4l of the negative-going K2 pulse developed across the resistor R98 and supplied via condenser C4i from the pulse generator PG. The 8 micro-seconds impulse of carrier frequency is ampliied at valve `48 and transmitted through the output transformer of that valve to the transmitting quartz plate of the delay line DL. The delay line is constructed to have a delay of ninety-six micro-seconds and, at the end of this time, the impulse of carrier frequency emerges, greatly attenuated, from the receiving quartz plate. After passage through the delay line, the impulse is amplied by valves 49 and 50 and a part of the power rectified at diodes D4 and D5, to produce a positive-going impulse across the resistor R45 suiiicient to raise the control grid of valve 45 from cut-ofi? potential to which it is biassed through the potentiometer R49 and R54,

connected between earth and negative potentialv V5, to substantially earth potential. Valve 45 has the cathode earthed and the suppressor grid pulsed continuously through condenser C42 by pulses of amplitude substantially equal to the cut-off bias voltage V which is applied to the suppressor grid of valve 45 Via REI. Since the delay in the supersonic line DL is 96 kmicro-seconde it follows that valve 45 will be switched on at the control grid beiore'the leading .edge of'an individual KI impulse occursJatthe suppressor grid. and switched 'o lagain iat -thecontrolgrid` after 'the said leading edge. Alsingle impulsek is therefore gated into the anode circuit-of valve 45 and develops a negative-going limpulse across resistor R40 exactly one .hundred micro-seconds after the previous impulse across the resistor. Thus trigger valves 46 and 41 'are :againchanged over and then Vrestored by a K2 .impulse and the process continuing as above, it will ibe seen that circulation having been `started will .continue until deliberately interrupted provided the delay of the delay Yline remains within the Arange 92-100 micro-seconds.

The negative-going pulse which is "thus developed on the common anode resistor R40, being developed -in the first instance by a channel impulse applied to the'control grid vof valve 4i) and subsequently byimpulses gated through valve '45 from the KI pulse at the 'suppressor of valve 45 is necessarily synchronous with, and comprises impulses which endure for the same periods as the channel impulses which correspond to the particular subscriber who has engaged his line. This negative-going pulse is employed to open the communication lgates in order to admit into the line finder the appropriate channel pulse on the common signal circuit G and lto emit from the line finder a synchronous channel pulse into the common signal circuit R. The communication gates comprise the pairs-of valves-4`l and 43, 42 and 44. Valves 4i and 42'arevampli`ers, valve 4| having a control grid circuit connected `Awith the common signal circuit Gand individual anode circuit while valve 42 has an individual control grid circuit and anode circuit Vconnected Yto thecommon signal circuit R. In both cases theanodel supply potential is earth, while the"cathode'sup ply potential V5 is negative with respect to earth. With no input signal and with valve 43 removed, the control grid of valve4l is rbiased to cut-oli` by means of the `potential V6, see Fig. 3a, andthe cathode resistor R41 'is so chosen that a positivegoing channel pulse -of standard amplitude will raise the potential on the 'control grid into the range of potentials within which valve 4| ampli- :des saitsiactorily. The control gridof valve 42 is also normally biased to cut off, 'the potential on the earth phantom of the return pair being, under these conditions; such in conjunction with the potential V8 and the potentiometer RSI and R62 as to produce the necessary bias'via the secondary winding of the transformer T5. vWhen, as will be seen, the called subscriber answers, the potential of the earth phantom of pair SR is raised to a level suicient to ensure that the rectifier W8 conducts and so clamps the bias of the control grid of 'valve 4-2 at the potential V5 in which condition valve 42 amplies satisfactorily. In addition to the switching operations performed on valves 4i and 42 by the changes in control grid bias above described, both valves are normally rendered non-conducting by the 'cathode-biasing action of the valves 43-and 44. Thesevalves have their anodes connected direct to the supply pot-- tential V! and theircontrol'grids biased via resistor R46 and supply potentia1 Vl which is positive with respect to V5. The cathodes of valves 43 and 44 are'conne'oted tothe cathodes'of valves 4l and 42 respectively. Such is the Vbias voltageVl that, in the absenceofany potential superimposed on it, the cathodes of valves 43 and 44 and therefore those of 4I and 42 are held so far positive with resp'ectlto `V5 'that valve 4l is maintained out olf even when 'th'ere'are incident at the controlgridof that valve channel impulsesv of the maximum amplitude cwhich*may "be exh" aeeasoopected. When, however, a line nder is engaged and is, therefore, developing across resistor R40 a negative going pulse synchronous with the channel pulse to be selected, each impulse of this negative-going pulse is superimposed, via condenser C4'|, on the bias applied to the control grids of valves 43 and 44, which valves are consequently cut olf thereby enabling valves 4| and 42 to amplify during the duration of the impulse. These are the means according to the invention by which selection of a wanted channel is effected.

The selected channel pulse which is gated through valve 4| is then passed through a lowpass filter 400 and amplifier 500 of similar constructions to those ||2 and ||3' described in relation to the subscribers exchange apparatus, and the voice frequencies so extracted are transmitted via the transformer T4 to the pair SG.

Another feature is the means by which pulse circulation is started in the line-finder. If reference is made to Fig. 3a it will be observed that channel impulses, after polarity inversion at valve 3|, are served to the control grids of both cathode followers 32 and 33. The cathode follower 32 applies signals to the common calling pulse circuit, lead C, and it will be observed that although normally biased close to cut-off by potential V4 via resistors R9 and R'I, at which bias valve 32 satisfactorily ampliiies the channel pulse, the application of an impulse of alternating current to the pairs PS will, after said pulse has been amplied, at transformer T6 and Valve 31, and rectified at transformer T3 and diodes D2 and D3,

result in superimposing a. negative-going impulse f amplitude to be expected. The pair PS is common to all line nders, which communicate to it, via decoupling resistors such as R63 and R64 shown in Fig. 3c, a proportion of the power of the 8 micro-second pulse of carrier frequency which emerges from the amplifier following the mercury delay line. Thus is the gate valve 32 prevented from passing to the common calling pulse circuit C any channel pulse which is already in circulation in a line finder.

It will now be apparent that when any subscriber originates a call the channel pulse allocated to him will appear on the calling highway C until he becomes connected to a line finder when by virtue of the above described means the said channel pulse will be barred from the common calling pulse circuit. Referring again to Fig. 3c the common calling pulse circuit C is -connected via condensers such as C40 to the control grids of valves such as valve 40 in each line finder. Valve 4) is the means by which an impulse appearing on the common calling pulse circuit may be injected into the circulation system of the line finder marked by the trunk marker LFA. As has been said, valve 40 has the anode connected via the anode resistor R40 to the supply potential VI, and the cathode to a tapping in the potentiometer comprising resistors R43, R44, and R45, said potentiometer being connected between the supply potentials VI and V5. A second tapping in the potentiometer is connected to the Contact X of the trunk marker the potentials VI and V and the resistors of the potentiometer being so designed that when a first selector is not marked by the trunk marker the cathode of valve 40 is held at a potential so far positive with respect to the control grid that valve 40 is cut off so that it is not affected by the maximum amplitude of channel impulse which may appear via condenser C40 and when a line finder is marked by the trunk marker cathode of valve 40 is brought to a potential, relative to the control grid such that valve 40 is then at cut-off point and is thus enabled to respond to a positive-going impulse applied via condenser C40. This potential of the cathode of valve 40 in the marked condition is stabilized at the supply potential V3 by the rectifier element W2 which conducts when the potential of lead X is held at earth potential. The control grid of valve 40 is normally biased to earth potential via resistors R4! and R42, a condition, however, which is immediately modified after impulse injection into the line nder has taken place, for a proportion of the power of the pulse of carrier frequency transmitted to the mercury delay line DL obtained via the potentiometer R52 and R53 is rectified by diode D9 and, consequently, a negative going potential is rapidly developed across resistor R42 thus biasing back the control grid of valve 40 sufciently to prevent the injection of a second channel in pulse, originated by a second calling subscriber, before the trunk marked LFA has moved on and so unmarked the selector. Resistor R42 is shunted by a condenser C43 which, in addition to providing smoothing for the rectifier D3, is made so large that the negative potential develop-ed across R42 decays only by a small amount between successive impulses of carrier frequency thus maintaining valve 40 cut-olf at the control grid whilst the line finder is held. This feature prevents injection into a line finder of impulses of more than one channel pulse.

r[he condenser C48 connected between the cathode of valve 40 and earth is provided to decouple the cathode resistance during the period of pulse injection.

Another feature of the arrangement consists in the transmission through the line finder of the calling and holding and clearing signals. When a line finder becomes connected to a calling subscriber as described above the negative-going potential developed across R42, in addition to closing the injection gate i. e. valve 40, is communicated to the control grid of the normally conducting valve 5| via condenser C44. The anode of valve 5| is connected to the supply potential Vl via resistor R63, the control grid of this valve to earth via resistor' R54 and the cathode directly to earth. The negative-going potential communicated via condenser C44 is sufficient to cutoff valve 5| and, such is the time constant at the control grid of valve 5|, the negative potential there decays but slowly and is in fact arrested in its decay by the action of the channel pulse gated through valve 4| continually discharging the condenser C44 via the diode Dl and resistor R55. Resistor R55 is provided to limit the current drawn by the diode Dl to a value insucient to cause appreciable distortion to the channel pulse gated through valve 4|. Valve 5| is therefore rapidly rendered non-conducting when the selector becomes engaged and is maintained in that condition by the presence of the channel pulse gated through valve 4|. If the calling subscriber were now to abandon the call by open-circuiting his line, the gated channel pulse would disappear and, valve 4| thereafter remaining non-conducting, valve 5| would become conducting after a period of time dependent on the time constant of the control grid circuit of valve 5|. This time constant is made such that valve 5| will not be .tronic Engineering=vol. 20, 1 No." "-42.45, zluly1948,

andin an: article entitled-Mercury DelayLines by T. K. Sharpless which appearedin the: periodical ,Electronics,lv dated November .1947, at pagel.

it will be appreciatedbyfthosezzskilled inithe art that practical .ditlcultiesxarise whenattempts'are madetoxconstruct amultiplicity of timingunits, `such asithe mercury filled delay line,. each so stablewithqrespect to the master timing, device, namely the KI. pulse, that y when started .said units will remainraccuratelysynchronous with the Klpulsefior along period. :It

vis possible, however,| and means willbe described to overcome these difficulties .by .employing a source of synchrcnising i pulses f and using these to re-'phase the circulating puise reach time it reaches theinjection point and so 'by preventing the cumulative eiiectofthedrift inthe delay, line, limit the Aerror tothat fof a singleotransmission through the line. In this condition, the delay stability or" the mercury ydelay line isadequate for the purpose required.

Valves t@ and i5 of aFigiScare the impulse injection and synchronising gates respectively u A negative-going impulse emitted from the common anode circuit, of these valvesis'arranged Ato ce communicatedtothe Suppressor grid of the normally conducting valve 46 thus changingr over the trigger valves -46`and141 fromthe rst position of stability to the second. The anode circuits of the triggervalves 46 and 41 are conu nected to a balanced static relay comprising the rectifying elements W4, ;W5, .Woand W1 and associated transformersv andi resistors in4 such a manner that when thetrgger is in thegjrst position of stability the attenuation between acarrier frequency oscillator C G and an amplier valve fl is very great and when the trigger is Ain the second position of stability;Saidattenuation is rendered small. In consequence; therefore, of changing over the trigger, carrierfrequency is admitted to the amplifier iR- until the trigger is changed back. This occurs 8 micros-seconds later by virtue or the incidenceat thesuppressor grid of valve lll of the-negative-goingKZ pulse developed across the resistor R98 and'supplied via condenser Ct! from thepulsergenerator PG. The 8 microeseconds impulse of carrier frequency is amplified at valve 43 and transmitted through the output transformer Vof that valve to the transmitting quartzplate of the. delay line DL. The delay line is constructedto have a delay of ninety-six micro-seconds and, at the end of this time, the impulse ,of carrier frequency emerges, greatly attenuated, from the receiving quartz plate. After passage through the -delay line, the impulse is amplified byvalves 49and'50fand1a part of the power rectied at diodes D vvand D5, to produce a positivevgoing, impulse across the resistor R48 sufficient to raise the control grid of valve from cut-oli potential to `which it is biassed through the potentiometer R49 -and R50, connected between earth and'A negative potential V5, to substantially earth potential. Valveii has the cathode earthed and the suppressor grid ypulsed continuously, through.4 condenser C42 by pulses of amplitude substantially equal to the cut-oil bias voltage V5 which isapplied to the ysuppressor grid of valve -45 via R51. Since the delay in the supersonic line DL is 96 micro-seconds it fcllowsthat valve 45 Willbe switched'on at the control grid beiorethe vleadingiedgeofv an individual KI impulse: occurs-g at the-suppressor fgrid, .f and switched: ott iagarolzat Ithe; control grid 75 after, thezsaidz'leading=-edge. v;-A^,S ing1eKI impulse is therefore' gated:l into;theanodeicircuit of .valve d5: and. develops va vnegative-going impulse across resistor VR40 exactlyone hundred micro-seconds after the previousgimpulse across'i thez resistor. Thus triggervalvesill and41: are:.again^changed over and thenfrestored byfat-KZ impulse andthe rprocess.;continuing fas; aboveg-it will be lseen that circulation having been fstarted will continue until deliberately interrupted z provided; the` delay or the delay line remains within the Vrange 2-100 micro-seconds. 1 :The negative-going; pulse which: .is v'thus ideveloped on lthe commonl anode vresistor lRltgbeing developed; in .the first instanceby a channerimpulse applied to the control grid of-fvalvezl and .subsequently byfimpulses gated? through valve d5 from therKl pulsefat':the-suppressor of valvei is necessarilysynchronous with, and. comprises impulses L which endure :for 1 thesame periods. as the channel vimpulses fwhich correspond to: the particular subscriber 1 who `has i engagedv his line. rThis negative-going `pulse`is--em1ciloyeol to-k open the communicationfg-ates-inorder to admit into the line nder the appropriate channel pulse-on the common signalcircuit.v G and toemit' from the line nder asynchronous channel pulseinto the common signalcircuitR. "The`| communication gates comprise' the lpairs yof valves-4| and-"43, l2 and M.. '.Valves'll 'and' Il?.1 are-amplifiers,v valve lll having a controlgridvcircuit connectediiwith the common signal circuit G andindividual anode circuit while vali/e142 `has'la,-1:i"i-ndividual control grid circuit and'anode circuit connected tothe common signal circuit R. :In both cases the anode supply potential' isearth;A while the cathode-supply potential V5 is -negativewithfrespect toearth. With no input signaland Awith va1vef43v-removed, the control'grid of valve Mwisbiasedto cut-'off by means of the potential V6; see Fig.' 3a; and the cathode resistorv R41 -isso chosen that apositivegoing channel .pulse of standard :amplitude will raise the potential'on' the control grid into the range of potentials Within which valve 4I amplies saitsfactorily. The control'vgridiof valve-42 is also normallyfbiased'to cut oi-thepotential on theA earth phantomA of'- thereturnfpair being, under these conditions; such-in onjunctionwith the potential V8 and the'potentiometer RSP- and R62 as to produce the necessaryI bias v iathe secondary windingof the transformeriTS- --When,as vwill be seen, thecalledsubscriber answers,` the potential of theearth-phantom of pair lSiRis raised to a level sufficient to ensure that thereotiler W3 conducts -andsorclampsthebiassof the control grid of lvalvef--at the-potentia1iV5fin vwhich conditionrvalve ampliessatisfactorily. In addition to theswitching operations performed on valves lll and'42 by thechanges in control grid bias above described, both-va1ves vare normally rendered Vnon-conducting by' the vcathode-biasing action ofthe valvesfl3 andll4. VThese valves have theiranodes connected direct-'to thevsupplyl-potential' V I and their controlgrids biased via-'resistor R46 A,and-supplypotentiell-Vl which is positive with respect to V5. The cathodes of valves 43 and ed are-connected to-the cathodesiof Valves 4l and-"42 respectively. jSuch is the'V bias voltage Vl that, in' the vabsence 'of anypotential superimposed on it,` thecathodesof/valves 43 and .M and thereforethose-offdl -andf'42are-he1d so far positivek with respect tofVS-t-hat valve- 4I is maintained cutAoiieven-when there are incident at .the control grid of vtl'iatvalve'channel impulses of; the. maximuzrwamplitudefwhichrmay"l bef-expected. When, however, a line finder ls engaged and is, therefore, developing across resistor R40 a negative going pulse synchronous with the channel pulse to be selected, each impulse of this negative-going pulse is superimposed, via condenser C4'I, on the bias applied to the control grids of valves 43 and 44, which valves are consequently cut olf thereby enabling valves 4| and 42 to amplify during the duration of the impulse. These are the means according to the invention by which selection of a Wanted channel is effected.

The selected channel pulse which is gated through valve 4| is then passed through a lowpass filter 400 and amplifier 500 of similar constructions to those ||2' and ||3 described in relation to the subscribers exchange apparatus, and the voice frequencies so extracted are transmitted via the transformer T4 to the pair SG.

Another feature is the means by which pulse circulation is started in the line-finder. If reference is made to Fig. 3a it will be observed that channel impulses, after polarity inversion at valve 3 I, are served to the control grids of both cathode followers 32 and 33. The cathode follower 32 applies signals to the common calling pulse circuit, lead C, and it will be observed that although normally biased close to cut-off by potential V4 via resistors R9 and R1, at which bias valve 32 satisfactorily amplies the channel pulse, the application of an impulse of alternating current to the pairs PS will, after said pulse has been am pliiied, at transformer T6 and valve 31, and rectied at transformer T3 and diodes D2 and D3, result in superimposing a negative-going impulse on the bias of valve 32. The amplitude of the negative-going impulse so obtained is sutlicient to render valve 32 non-conducting even when served with a channel impulse of the maximum amplitude to be expected. The pair PS is common to all line nders, which communicate to it, via decoupling resistors such as R63 and R64 shown in Fig. 3c, a proportion of the power of the 8 micro-second pulse of carrier frequency which emerges from the amplifier following the mercury delay line. Thus is the gate valve 32 prevented from passing to the common calling pulse circuit C any channel pulse which is already in circulation in a line finder.

It will now be apparent that when any subscriber originates a call the channel pulse allocated to him will appear on the calling highway C until he becomes connected to a line finder when by virtue of the above described means the said channel pulse will be barred from the common calling pulse circuit. Referring again to Fig. 3c the common calling pulse circuit C is connected Via condensers such as C40 to the control grids of valves such as valve 40 in each line finder. Valve 40 is the means by which an impulse appearing on the common calling pulse circuit may be injected into the circulation system of the line nder marked by the trunk marker LFA. As has been said, valve 40 has the anode connected via the anode resistor R40 to the supply potential Vl, and the cathode to a tapping in the potentiometer comprising resistors R43, R44, and R45, said potentiometer being connected between the supply potentials V| and V5. A second tapping in the potentiometer is connected to the contact X of the trunk marker the potentials V| and V and the resistors of the potentiometer being so designed that when a rst selector is not marked by the trunk marker the cathode of valve 40 is held at a potential so far positive with respect to the control grid that valve 40 is cut olf so that lt is not affected by the maximum amplitude of channel impulse which may appear via condenser C40 and when a line finder is marked by the trunk marker cathode of valve 40 is brought to a potential, relative to the control grid such that valve 40 is then at cut-olf point and is thus enabled to respond to a positive-going impulse applied via condenser C40. This potential of the cathode of valve 40 in the marked condition is stabilized at the supply potential V3 by the rectifier element W2 which conducts when the potential of lead X is held at earth potential. The control grid of valve 40 is normally biased to earth potential via resistors Rill and R42, a condition, however, which is immediately modified after impulse injection into the line nder has taken place, for a proportion of the power of the pulse of carrier frequency transmitted to the mercury delay line DL obtained via the potentiometer R52 and R53 is rectied by diode D9 and, consequently, a negative going potential is rapidly developed across resistor R42 thus biasing back the control grid of valve 40 sufficiently to prevent the injection of a second channel in pulse, originated by a second calling subscriber, before the trunk marked LFA has moved on and so unmarked the selector. Resistor R42 is shunted by a condenser C43 which, in addition to providing smoothing for the rectilier D9, is made so large that the negative potential developed across R42 decays only by a small amount between successive impulses of carrier frequency thus maintaining valve 40 cut-olf at the control grid whilst the line nder is held. This feature prevents injection into a line nder of impulses of more than one channel pulse.

The condenser C48 connected between the cathode of valve 40 and earth is provided to decouple the cathode resistance during the period of pulse injection.

Another feature of the arrangement consists in the transmission through the line iinder of the calling and holding and clearing signals. When a line finder becomes connected to a calling subscriber as described above the negative-going potential developed across R42, in addition to closing the injection gate i. e. valve 40, is communicated to the control grid of the normally con ducting valve 5| via condenser C44. The anode of valve 5| is connected to the supply potential V| via resistor R63, the control grid of this valve to earth via resistor R54 and the cathode directly to earth. The negative-going potential communicated via condenser C44 is sufficient to cutoff valve 5| and, such is the time constant at the control grid of valve 5|, the negative potential there decays but slowly and is in fact arrested in its decay by the action of the channel pulse gated through valve 4| continually discharging the condenser C44 via the diode Dl and resistor R55. Resistor R55 is provided to limit the current drawn by the diode Dl to a value insufficient to cause appreciable distortion to the channel pulse gated through valve 4|. Valve 5| is therefore rapidly rendered non-conducting when the selector becomes engaged and is maintained in that condition by the presence of the channel pulse gated through Valve 4|. If the calling subscriber were now to abandon the call by open-circuiting his line, the gated channel pulse would disappear and, valve 4| thereafter remaining non-conducting, valve 5| would become conducting after a period of time dependent on the time constant of the control grid circuit of valve 5|. This time constant is made such that valve 5| will not be rendered conducting during the temporary absence of the selected channel pulse caused by the standard breaks in a dialled signal but will be rendered conducting if the said pulses remain absent for an appreciably longer period, thereby identifying the clearing condition. rEhe rise in anode potential of valve l when the lineinder becomes engaged is used to pass forward over the pair SG a positive-going potential, obtained via the potentiometer R583 and Rii, which is connected, between the anode of valve 5l and the supply potential V5; this positive-going potential, as will be seen causes the calling signal to be applied to the called subscribers line. In addition the said rise in potential is communicated Via condenser Ci to the control grid potentiometer of the trigger valve il Where it is absorbed without disturbing the trigger by the rectifier element W3. When the calling subscriber eventually clears and valve 5l becomes conducting the resulting negative-going potential developed at the anode of 5i is communicated to the control grid of valve il and there holds valve di cut-oil` for a period greater than the multiplex period and by so interrupting circulation, releases the line finder.

Another feature of the line nnder consists in the provision or" means for the detection of dialled impulses. To the anode of the valve il is connected, via a resistor R55, a diode DS by means of which, when the line finder beco es engaged, a negative potential is developed on the control grid of valve 52 sufficient to render that valve nonconducting so that the normally operated relay .a is released. The cathode of valve r2 is earthed and the control grid also earthed via resistor R57. Resistor R53 is shunted by a condenser Cl, the time constant of this combination being such that valve 52 is switched, and relay A satisfactorily operated, by each break caused by dialling. The purpose of the resistor R55 is similar to that of to limit the flow or current through the diode D8.

Before describing the purpose of the mechanically operated switch and relays in the line nnder circuit reference will be made to the selectors. Examination of Fig. 3d shows that each selector is connected to the common signal circuits G and R of the multiplex via communication gates lili), :i3d and. which are similar to the gates i, and d2 and ill of the line finders. more, the circulating system and pulse injection arrangements are also similar to those in the line finders. A dille-rence, however, occurs in the release arrangements.

rIhe control grid potentiometer of the normally non-conducting valve, dit, oi the trigger valves @le in a selector is connected at one to the anode of the other trigger valve it at the other end to the tapping point of a second potentiometer' and R55. This second potentiometer is connected between a supply potential V6 which is negative with respect to V5, and the earth phantom of the pair SG. When the said earth phantom is raised in potential by be calling and holding signal passed forward the associated line finder the tapping point of the potentiometer Re? and R56 is raised substantially to the potential J5 in which condition trigger able to opera-te satisfactorily but when the said calling and holding signal is removed the potential at the tapping point of R63' and is so reduced that the trigger is unable to operate.

Valve Fig. Bd has its control grid connected to the earth phantom of the pair SG and its Furtheri cathode to the supply potential V3. The anode of valve 54 is connected to a tapping in the potentiometer RMB, R449, R559 controlling the injection gate d as is the lead X connected to the similar potentiometer Rll, Rllfi, R555, in the line nder circuit. In the release condition of the selector valve 5i is non-conducting and the cathode of the injection gate valve so! is 'then so positive with respect to the associated control grid that injection is prevented. When, however, the calling and holding signal is extended from the line finder, valve 54 becomes conducting and so modiies the cathode bias of the injection gate that injection is then permitted.

It is now clear that if a selector is prepared by the receipt of a calling and holding signal from the associated line finder, then any positive-going channel impulse of correct amplitude appearing on the common marking pulse circuit SM will be injected into and thereafter circulated within the said selector which by virtue of the transmit and receive gates dit, d2@ now becomes connected to the subscriber who has been allocated a channel pulse synchronous with that appearing on the circuit SM and injected into the selector at valve 40|.

The calling and holding signal, being present on the earth phantom of the pair SG, raises the grid bias on the valve siZ in Fig. 3d to the operating point, thus causing said gate to emit a channel pulse into the common signal circuit R and this channel pulse, being synchronous with that of a particular subscriber, will be distributed to 'nim and by causing the release of the relay S, in his exchange apparatus, cause him to be called. When the called subscriber answers he causes to be emitted into the common signal circuit G connected with the selector of Fig. 3d the channel pulse allocated' to him, which is thus gated through the valve lli of the appropriate selector and by rendering valve 5i@ non-conducting in manner similar to thatdescribed with reference to the line finder of Fig. 3c causes a rise in potential of the earth phantom of the pair SR. This, as has been described, results in the release of the S relay of the calling subscriber thus giving the answering signal.

When a calling subscriber becomes connected to a line nder and dials the number of the Wanted subscriber, the channel pulse appropriate to the Wanted subscriber is made to appear on the common marking pulse circuit SM. Referring to Fig. 3a, valves 2l to St have a common anode resistor R58 and are connected through the resistor and the Cle-coupling resistance and condenser combination R58 and C59 to the supply potential Vl. The cathodes of valves 2i to 30 are earthed and the control grids, each joined via a resistor to a supply potential V13, negative with respect to earth sufficiently to cut off the associated valves. The control grids are also multpled, leads Ml to Mill, over the bank contacts D82 of digit switches, DS, of which one is provided for each line iinder. Valves 2l to 3d have the suppressor grids directly connected to those of the multiplex modulator valves l to ill, respectively, and if the control grid potential of valve 2| is raised by the application of earth potential to the corresponding bank contact in any of the digit switches, then valve 2l will emit a negativegoing pulse into the common anode circuit, the impulse of this pulse being synchronous with the impulses of the channel pulse of the :subscriber associated with Valves l and il.

vSimilarly by the application of earth potential to any of the leads Ml to lvIlIl may the corresponding channel pulses be made to appear in the common anode circuit of valves 2l to 3b. Impulses appearing in this anode circuit are rst polaritv -inverted by the ampliier 38 and then communicated via the cathode follower 39 to the circuit SM. In order to connect a selector to a particular subscriber it is necessary to apply earth potential to the appropriate lead MI to M Ill and at the same time to extend the calling and holding signal from the associated line finder.

Reference to Fig. 3d shows that the eight micro-second impulses of carrier frequency emerging from the delay line output ampliiier of the engaged selectors, in addition to providing the rectiiied impulse necessary for the maintenance of circulation, are communicated via leads PSO and decoupling resistors to the common pulse suppression circuit PS. As has been described the similar pulse circulating in the line finders are also made to appear on the circuit PS, and it will be clear then that the transformer TS, served by the circuit PS, will transmit the eight micro-second, carrier-frequency Versions of the channel pulses of all the engaged subscribers in the exchange. These pulses are rectified by diodes D2 and D3 to produce negative-going pulses across the resistor R9 and, since RS is part of the total grid leak resistance of the cathode follower valve 39 and because the impulses of these impulses occur in advance of the related channel pulses and endure until after said channel impulses have passed, they so modify the grid bias of valve 3S as to prevent the transmission into the circuit SM of any channel pulse already engaged. This constitutes the means whereby a called subscribers line is tested for the engaged condition before being connected to a selector.

Referring again to Fig. 3c, when a subscriber becomes connected to a linender the normally non-conducting valve 53 is rendered conducting by virtue of the connection between the control grid of that Valve and the earth phantom of the pair SG. Relay E operates and at contacts El extends earth potential over lead W to the trunk marker uniselector banks. In addition to the above the act of engaging a line nder causes release of the normally operated impulse accepting relay A as already described. After a short delay the slow-to-release guard relay B operates over contacts AI and operates the common cutoff relay CO in the trunk marker by closing contacts BI thereby preventing at contacts COI the marking over lead X of a line finder to take the next call. As will be seen, relay CO remains operated during the time taken to make the connection between two subscribers. Operation of relay B also open-circuits the circuit including the homing arc, DS3 at contacts B2, prepares the digit switch driving circuit at contacts B3 and prepares the hold circuit of relay MF at contacts B4. Onthe irst break in the dialled train relay A operates and completes the driving circuit ofr digit switch driving magnet DS at contacts AI Via contacts B3, the home contact of the arc DSI, series relay C to the driving magnet DS. Slow-to-release relay C operates, closes contacts C3 and relay CC operates. The switch steps at the end of the dial break when relay A is again released. Subsequent breaks of the dial train maintain relay C operated and step the switch via the arc DSI and the contact C4. At the end of the train of impulses relay C releases and somewhat later relay CC releases. In the interval, i. e. the release time of relay CC, relay MF operates via contacts CI and CC and locks over contacts MFZ and B4. Operation of relay MF initiates release of relay CO at contacts MFI. Also during the release time of relay CC earth potential is extended via contacts C2, CC2 and the arc DS2 to the appropriate lead MI to MIU. Thus depending on the digit dialled is connection made to the wanted subscriber the marking pulse of the called subscriber, if free, being injected into the selector as described. In the event of that subscriber being engaged, no pulse appears on the circuit SM during the release time of relay C and in consequence no connection is made. When relay C releases, the driving circuit is opened at contacts C4 so that any further dialling is ineffective.

Release of the connection is under the control of the calling subscriber for when he open circuits his line the channel pulse gated through the valve 4I of the line nder, reduces to zero amplitude. Relay A then operates and initiates the release of slow relay B. Valve 5I becomes conducting and the resulting fall in potential at the anode produces via condenser C46 a negative-going impulse, at the control grid of valve 4l', which, as has been described prevents operation of the trigger and so stops impulse circulation. In addition the fall in potential at the anode of valve 5| removes the calling and holding condition from the earth phantom of the pair SG and so stops impulse circulation in the selector, and also by rendering valve 53 non-conducting releases relay E and so by removing earth potential from lead W renders the line finder available to handle another call. Meanwhile relay B releases, releasing relay MF at contact B4 and at contacts B2 completing the homing circuit of the digit switch.

Referring again to Figs. 3a and 3c, it will be clear that these figures disclose a switch having a selector side (Fig. 3c, right of commons C, G and R), a bank side (Fig. 3a, left of commons C, G and R) and a common bus section (C, G and R) therebetween. Also that the selector side comprising a plurality of link circuits each presenting outwardly extending main and signal entry trunk means (SR and phantom to 42, LFA to 40) and main and signal exit trunk means (4I to SG and phantom), while the bank side comprising a plurality of bank circuits each presenting outwardly extending main and signal entry trunk means (Fig. 3a, WI, R2 leads to gates I-IO) and main and signal exit trunk means (Fig. 3a, left from tubes Ii-Zfi). Further, that each selector side and bank side circuit has at its inner end exit and entry trunk gating means (e. g. gates 4i and 42, and gates I-Ill and I I-Z) the gating means of the several bank circuits (I-I and II-23 being operable in time division multiplex sequence, and the gating means of each of the selector circuits being operable in synchrony with the time division channel of any one of the bank circuits. Also that the signals from any of the bank side circuits gated to the common bus G at I-Ill are presented to the exit gates 4I of all the selector side circuits, while those from all the selector side circuits, gated to the common bus at 42, are presented to the exit gates II-Z of all the bank circuits. Clearly, also, each bank circuit comprises means (the cathode has connections in tubes I-IU) responsive to a signal entering through that bank side circuit (WI-R2) for initiating production of channel pulses on common bus G on its own time division channel, and Sach selector side circuit comprises means responsive to a vsignal entering through its signal entry trunk means (LFA to til) and to channel pulses of a bank circuit (over common ons C to 50) for operating its gating means iiid2 in vsynchrony with such channel pulses, the switch further comprising call isolating means, shown as the pulse suppression means 32 associated with the bus C, that blocks from the signal responsive vmeans t@ of all vselector side circuits the channel pulses of a signal that has already engaged one selector side circuit.

Still referring to the switch of Figs. 3a and 3c, it is clear that the pulse circulating circuit through DL. constitutes a synchronous channel pulse generator, having a pulse injection ga 'e it conditioned by the marking signal from .LFA to admit a channel pulse from C, and connected for operating the selector side gates fil and #l2 in synchrony therewith, and further including means shown as the circuit from D@ through RM for blocking its pulse injection gate when it `is generating pulses synchronous with those of a communicating channel.

Referring now to Figs. 3a and 3d, it will be clear that these gures together disclose a second switch generally the same as that just discussed, having generally similar selector side circuits SG and SR and their phantoms, and generally similar banlc side circuits (common to 'the first switch in the illustrative `embodiment oi Figs. 3a through 3d) and that the two switches together, with their selector sides interconnected through SG and SR and their phantoms, and with controlling markers LFA and SM, constitute a switching system of an exchange.

As will further he evident to those skilled in the art, the embodiments herein set forth are illustrative and not restrictive of the invention, the scope of which is dened in the appended claims, and all modications that vcome within the meaning and range of equivalency of the claims are intended to be included therein.

I claim:

1. Automatic telephone equipment, comprising a common transmit circuit and a common receive circuit; a plurality of subscribers stations including lines each comprising a transmit trunk and a `eceive trunk; time division multiplex apparatus comprising a pulse transmit gate for each transmit trunk, said gates interposed between said trunks and said common transmit circuit, a corresponding pulse receive gate for each of said receive trunks, said receive gates being interposed between said common receive circuit and said receive trunks, a common called channel marker circuit and connected thereto a called channel marker gate for each subscriber channel; said time division multiplex. apparatus further including pulse generating means for simultaneously unblocking the transmit, receive and channel marker gates of the respective suhscribers in time spaced order; said pulse transmit gates comprising second hlocks respectively removed by energiaation or the transmit trunks connected thereto; a plurality of line finders, said line finders each comprising a pulse :lnjector and transmit and receive gates for the calling channel; a calling channel marker circuit extending from said common transmit circuit and commoned to said pulse injectors, said common transmit circuit also being comisiones to the transmit gates of said line finders, and the receive gates of said line finders being connected to said common receive circuit; means for marking the pulse injector of an idle line nder for receiving animpulse ifrom said calling channel marker, impulse circulating means responsive to the so-received impulse 'and to the pulse generating means for opening the calling channel gates of said line .finder :in synchrcny with the pulse channel `impulses oi a calling subscribers station; said subscribers vstations comprising make and break dial equipment, each line finder comprising digit switch means responsive to the dial Yinterrupted calling signal passed by the transmit gate of its `associated line finder, said digit switch removing a block from the called channel .marker gate of the called subscriber; said line nders each having la selector associated therewith and 'said selectors each comVm prising a pulse injector and transmit and receive gates for the called channel; said selector puise injectors being commoned to said common called channel marker circuit and said selector transmit and receive gates :being commoned, espectively, to common transmit and receive circuits; each of .said line ,nders comprising means ier demodulating kspeech components of the signals passed by the transmit gate lof said line and means `for connecting the demodulated output thereof to the receive gate oi its associated selector; :said selectors each comprising impulse circulating means responsive to the called chanw nel marker admitted through its injector from said called channel marker circuit and to the pulse generator -for opening the receive and transmit gates thereof in synchrony with the pulse channel of va called subscribers station; and said selectors each `comprising means for demodulating 'speech components of the signals passed by the transmit gate of Asaid selector and means for connecting the demodulated output thereof to the receive gate ci its associated line nder; whereby the pulse modalated signal produced only by a calling station and said time division multiples; apparatus is transmitted to an idle line finder gated thereby on the time division channel of the calling station, the gated calling signal being then applied to select the time division channel for the called subscriber and to provide de modulated components that are pulse gated on the so-selected called subscribers time division channel; and whereby the pulse modulated signal produced only by the answering called sta ed hv the vassociated selector on the called stations time div'ion channel and is applied to produce demodulated speech components that are pulse gated on the calling suhscribers time division channel.

2. Automatic telephone equipment according to claim l, in which the impulse circulating means of each line finder comprises inea-ns blocking its pulse injector despite continued marking thereof, as soon as an impulse has heen received therethrough.

3. Automatic telephone equipment accordi to claim l, in which the common calling char marker circuit comprises a pulse suppresL gate and means responsive to circulation o? pulses in the impulse circulating circuits et said line nders for suppressing transmission through said pulse suppression gate oi further impulses of a calling signal that has engaged a line 4. Automatic telephone equipment according to claim 1, including means for generating a long impulse on cessation of marking signals as a clearing signal, and means responsive to said clearing signal to terminate the operation of said impulse circulating means.

5. Automatic telephone equipment according to claim 4, in which each line finder further comprises timing circuit means for differentiating between dialing and clearing signals gated thereto by the transmit gate of such line nnder.

6. Automatic telephone equipment according to claim l, in which each of said line finders comprises means responsive to engagement of the line finders by a calling channel for producing a further signal and means for applying said further signal to the receive gate of its associated selector, and in which said selector receive gate is inoperable to pulse gate on the called subscribers time division channel the demodulated components of a calling signal in the absence of application of said further signal thereto.

7. Automatic telephone equipment according to claim 6, in which said further signal producing means produces a continuous signal that, for the duration thereof, continuously maintains said selector receive gate operable to pulse gate said demodulated calling signal components.

8. Automatic telephone equipment according to claim 1, in which each of said selectors comprises means responsive to an answer on the called channel for producing a further signal and means for applying said further signal to the receive gate of its associated line finder, and in which said line finder receive gate is inoperable to pulse gate on the calling subscribers time division channel the dernodulated components of the answering stations signal in the absence of application of said further signal thereto.

9. Automatic telephone equipment according to claim 8, in which said further signal producing means produces a continuous signal that, for the duration thereof, continuously maintains said line finder receive gate operable to pulse gate said demodulated answering signal componente.

10. Automatic telephone equipment according to claim l, in which each of said line finder pulse injectors comprises means for blocking its reception of an impulse from the calling channel marker circuit and in which the line inder marking means renders inoperative the blocking means of the respective pulse injector to which it is connected, and further comprising a signal generator responsive to engagement or" such line finder and means operable thereby to transfer connection of the line iinder marking means to another line finder to thus restore the operation of the pulse injector blocking means cf the line nder previously marked thereby.

ll. A switch having a selector side, a bank side, and a common bus section therebetween; the selector side comprising a plurality of circuits each presenting outwardly extending main and signal entry trunk means and main and signal exit trunk means for conveying signals to and from the switch, the bank side comprising a plurality of bank circuits each presenting outwardly extending main and signal entry trunk means and main and signal exit trunk means for conveying signals to and from the switch, and each of said seiector circuits and bank circuits having at its inner end exit and entry trunk gating means; the gating means of the several bank circuits being operable in time division multiplex sequence for producing signal puises on different time division channels at the inner end of each bank circuit, and the gating means of the several selector circuits each being operable on any selected one of the said time division channels; the common bus section comprising common bus means for receiving gated signals from any number of the bank circuits, each as channel pulses on its own time division channel, and presenting the same to the exit gating means oi all the selector circuits, and the bus section further comprising common bus means for receiving gated signals from any number of the selector circuits each as channel pulses on its selected time division channel, and presenting the same to exit gating means of all the bank circuits; each bank circuit comprising means responsive to a signal entering therethrough for initiating the produc tion of channel pulses on its own time division channel; each selector circuit comprising means responsive to a signal entering through its signal entry trunk means and to channel pulses of a bank circuit for operating the gating means of said selector circuit in synchrony with the channel pulses of the initiating bank circuit, thereby to establish communication between the initiating bank and selector circuits being synchronously gated to the common bus means, and said switch comprising call isolating means operable for the duration of such communication between a particular one of the bank side circuits and a particular one of the selector side circuits for preventing other selector side circuits from responding to and synchronizing with the channel pulses of the particular bank circuit.

12. A switch according to claim 1l, in which the means responsive to a signal entering through signal entry trunk means of a selector circuit and to channel pulses of a bank circuit, comprises a synchronous channel pulse generator having a pulse injection gate conditioned, by the signal entering through said signal entry trunk means of the selector circuit, to admit a bank circuit derived channel pulse for initiating operation of the pulse generator in synchrony with the initiating bank circuit pulse channel, said puise generator being connected for operating the gating means of the selector circuit in synchrony with that of the bank circuit from which the initiating pulse was derived, and in which each selector circuit further comprises means responsive to operation of its synchronous channel pulse generator for blocking its pulse injection gate.

13. A switch according to claim 12 in which the synchronous pulse generator comprises a com plete pulse circulating circuit excluding the pulse injection gate.

14. A switch according to claim l2 in which the synchronous pulse generator comprises a complete pulse circulating circuit excluding the pulse injection gate and including a delay line.

15. A switch according to claim ifi in which the means for blocking the pulse injection gate is connected to said circulating circuit ahead of said delay line.

16. A switch according to claim 1l, in which the common bus means comprises a rst bus that presents the channel pulses from the bank circuits to the exit trunk gating means or' all selectors and a second bus that presents the channel pulses from the bank circuits to the pulse injection gates of the synchronous pulse generators; and in which said call isolating means comprises pulse suppression means in said second bus and means responsive to generation or synchronous pulses in a selector side circuit for actuating said pulse suppression means for suppressing trans- 

